In clinical practice involving alterations in the level of consciousness, such as during the administration of sedatives or general anaesthetic agents, it is important to be able to quantify brain function. Most approaches rely upon the analysis of the brain's surface electrical activity, known as the electroencephalogram or EEG. In general the signal analysis method chosen is based on the statistical properties of the signal being analysed. The more closely matched the method used is to the signal properties, the more reliable, meaningful and accurate the resulting analysis will be. However these signal properties can only be known if the mechanisms and processes responsible for the generation of the signal are also known.
To date none of these analysis methods of the brain's rhythmic electrical activity have incorporated any details of the underlying physiological mechanisms responsible for its genesis. Therefore their ability to measure, and thus monitor, brain function in the clinical setting is limited.
This problem is overcome by the present invention which provides a more rational means of assessing and measuring brain function based on the detailed knowledge of the physiological mechanisms underlying the generation of the brain's surface rhythmic electrical activity.
The theory underlying the present invention considers the cortex of the brain as a single excitable spatial continuum of reciprocally connected excitatory and inhibitory neurons interacting by way of short-ranged (intra-cortical) and long-range (cortico-cortical) connections. As such, the brain is seen as a dynamically evolving entity rather than a synthetic processing unit like a computer.
Based on this theory, the characteristics of alpha rhythms arising as a consequence of the brain's neural connections can be closely represented by a mathematical model, and in particular, a fixed order auto-regressive moving average (“ARMA”) model. The present invention derives specific values for the moving average (“MA”) and auto-regressive (“AR”) orders for the ARMA model based on the electrocortical transfer function. The electrocortical transfer function describes in a mathematical form the origin of the EEG readings taken of a subject.
By applying EEG signals recorded from a subject to the fixed order ARMA model, coefficients can be obtained. To understand how these coefficients can be used to measure brain function, the equations defining the fixed order ARMA model are rewritten in the z-domain (complex domain) and are solved to obtain complex number solutions (called “poles”) that are mapped onto the z-plane. These poles represent the state of the brain at the specific point in time when the EEG signal was recorded. Variations in the EEG signal, such as that induced by applying sedatives to the subject, can be detected as variations of the mean location of one or more poles on the z-plane. These variations can be interpreted to measure brain function or to indicate changes in the state of the brain.
By using the brain assessment techniques of the invention, it is possible to monitor the state of a subject in various circumstances. For instance, the method of the invention can be used to monitor the vigilance or alertness of a subject when performing certain tasks such as driving vehicles of various types or controlling critical equipment. In applications of this type, the method can be applied locally so as to warn the driver or controller of a condition which is indicative of a loss of vigilance or alertness so that appropriate action can be taken. The monitoring could be carried out remotely as well as locally.
The method of the invention can be used to monitor a subject whilst sleeping so as to assess various stages in sleep of a subject. The results obtained can be used for determination and/or treatment of sleeping disorders.
Further, the invention can be used to monitor the state of anaesthesia of a patient. In this application it would be typical for the anaesthetist (or an operator) to obtain a display of poles in the said plane prior to administration of the anaesthetic. The method of the invention can then be continued after application of the anaesthetic so that the state of anaesthesia of the patient can be monitored as a function of time by reference to the movement of clusters of poles displayed on display equipment. This provides useful information to the anaesthetist regarding the state of anaesthesia of the subject.